Electroluminescence (EL) is the emission of light under electric-field excitation. Based on this mechanism, EL lamps are finding an increasing number of applications in the field of flat panel displays due to the growing demand for consumer electronic products, e.g., cellular phones and portable computing devices. EL lamps also provide uniform light emission independent of viewing angle and they are insensitive to mechanical shock and vibration. They can be easily DC-driven at 1.5-9 volts by using inverters that generate AC voltages of about 100-300 V (peak-to-peak) at frequencies of 50 to 1000 Hz.
The two major EL lamp constructions are generally referred to as thin film and thick film. Thin-film EL lamps are made by depositing alternating thin layers of dielectric materials, phosphors and conductive oxides on a glass substrate using a vapor deposition technique such as CVD. Thick-film lamps are made by suspending powdered materials in resinous materials and then applying the materials in layers onto a plastic film using conventional screen printing techniques. Hence, the thick-film EL lamps can be thin, flexible and rugged thereby making them suitable for a wider range of lighting applications.
The phosphors available for thick-film EL lamps are primarily comprised of zinc sulfide that has been doped with various activators, e.g., Cu, Au, Ag, Mn, Br, I, and Cl. Examples of these phosphors are described in U.S. Pat. Nos. 5,009,808, 5,702,643, 6,090,311, and 5,643,496. Examples of commercial EL phosphors include: OSRAM SYLVANIA Type 813, a blue-emitting ZnS:Cu phosphor, OSRAM SYLVANIA Type 723, a blue-green emitting ZnS:Cu,Cl phosphor and OSRAM SYLVANIA Type 523, a yellow-orange emitting ZnS:Cu,Mn phosphor. Typically, the individual particles of the EL phosphors are encapsulated with an inorganic coating in order improve their resistance to moisture-induced degradation. Examples of such coatings are described in U.S. Pat. Nos. 5,220,243, 5,244,750, 6,309,700, and 6,064,150.
For signage and backlighting involving multi-color informational displays, it is important to be able to generate a white light with a high color rendering index (CRI). However, the brightest electroluminescent phosphors emit primarily in the blue to green spectral region at wavelengths from about 400 nm to about 550 nm. This makes it difficult to obtain a high CRI, white emission with EL lamps especially since a satisfactory red-emitting electroluminescent phosphor for thick-film EL lamps has yet to be developed. At present, the two most prevalent white-emitting EL technologies involve (1) combining a blue-green emitting electroluminescent phosphor with a Rhodamine dye and (2) blending a blue-green-emitting electroluminescent phosphor with an orange-emitting ZnS:Cu,Mn electroluminescent phosphor. In the first case, the combination of the blue-green electroluminescent phosphor with the Rhodamine dye can produce a lamp which exhibits a CRI of about 79 and has an x color coordinate of about 0.27 and a y color coordinate of about 0.34. However, the Rhodamine dye gives the unlit lamp an undesirable pink color which makes it less desirable for signage applications. In the second case, the blended EL phosphors yield a CRI of only about 69 with an x color coordinate of about 0.34 and a y color coordinate of about 0.38. In addition, because two different EL phosphors are used, there can be problems with color shifts during the life of the lamp as the brightness of the phosphors degrade at different rates.
Chinese Patent Publication No. CN 1340590A describes mixing blue- and blue-green emitting EL phosphors with a cerium-activated yttrium aluminum garnet phosphor Y3Al5O12:Ce, (YAG:Ce). The YAG:Ce phosphor is a photoluminescent phosphor that is excited by wavelengths emitted by the blue and blue-green EL phosphors. The yellow emission from the YAG:Ce phosphor together with the blue- or blue-green emission from the EL phosphor creates a white light. However, the white light emitted by this combination lacks a significant red component and as a result has a CRI of less than about 70.